[swarming] fix deleting broken bot
[swarming] fix deleting broken bot

It may happen that a BotInfo exists, has a machine_lease, but the
MachineLease entity doesn't exist. In this case, the deletion should
still work and delete the bot.

Change-Id: I6261304b7534b962c37b5f548179631fb18cde9e
Reviewed-on: https://chromium-review.googlesource.com/c/1480441
Commit-Queue: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Commit-Queue: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Auto-Submit: Marc-Antoine Ruel <d2bf0fc09b08f7b0888b3ddab32b3e89c7122c8b@chromium.org>
Reviewed-by: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py